Idle speed adjustment/irregularity
 

I was hoping someone might have a suggestion regarding setting the idle speed on an 86 911.

The idle speed seems too low to me; per the tach, about 600 rpm (it feels like it too). Also, it erratically varies up and down a bit (while sitting).

I've tried the procedure set out in the Haynes manual and in the article on the subject here on PelicanParts; bridging the B and C connectors in the elec box in the engine compartment and adjusting the screw in the throttle body and then removing the bridge.

Once I set the idle speed and remove the bridge, the idle speed returns to a too low and erratic situation.

Any suggestions where I may be off base or where the car might have a prob?

You could have a gummed up (or defective Idle Stabilizer Valve). Try removing the ISV and spraying carb cleaner into the valve...shake it around and let carb cleaner dry. Reinstall the ISV and see if that helps.

Try running a bottle of Techron or Swepco 503 through your fuel tank. Some of your problem could be caused by partially clogged injectors.

Also, you might want to check your idle mixture (CO). You should do this adjustment with a CO meter. I set my '84 Carrera's CO between 3% and 3.5% and it runs very good.

Bruce

The idle speed on a Motronic system is set by the DME through the ISV. For an 86, I believe the speed is 800 RPM.

First of all, don't believe your tach. The DME will tell you what the idle speed is. Your tach may read high or low.

The bridging of contacts B and C closes the ISV. What you want to do is allow just enough bypass air to make the engine idle at DME idle speed with a warm engine and the ISV closed. This means the ISV really is only active on a warming engine.

What happens if there is too little bypass air is that the ISV will tend to oscilate, resulting in a varying idle speed.

Sounds like Carrera-talk to me. CIS idle adjustment is done with the thumb and index finger. Muuuhahahahahahaha.